1. Field of the Invention
The present invention relates to an ultrasonic ground speedometer utilizing Doppler effect which is for example adapted for detection of vehicle speed over the ground. Specifically to a speedometer which is capable of providing high accuracy and high responsiveness of ground speed measurement, irrespective of the state of road surface, namely, rough or smooth surfaces.
2. Description of the Prior Disclosure
Recently, there have been developed and proposed various ultrasonic ground speedometers. Such ground speedometers generally include an ultrasonic transmitter for outputting an ultrasonic wave, an ultrasonic receiver for receiving an ultrasonic wave reflected from the road surface and for generating a reflected ultrasonic wave signal, and an arithmetic circuit for deriving ground speed from a Doppler shift occurring in the output ultrasonic wave due to Doppler effect.
One such ultrasonic ground speedometer has been disclosed in Japanese Patent First Publication (Tokkai) Showa 60-76678.
However, in the above mentioned conventional ultrasonic ground speedometer, the wavelength of the output ultrasonic wave emitted from the ultrasonic transmitter would be freely selected from various values depending on characteristics of the ultrasonic transmitter and/or the ultrasonic receiver, beacuse of lack of sufficient analysis with regard to an optimal wavelength range for the output ultrasonic wave. For this reason, a sufficient S/N (signal-to-noise) ratio of a Doppler frequency signal representative of the Doppler shift between the output ultrasonic wave and the reflected ultrasonic wave would not be obtained. This results in relatively low accuracy of ground speed measurement.